Removes test format from url
Makes returning data from store the default behaviour for `process'
If the required format is 'txt', returns a content type of 'text/plain'
Migrates 'Plain text' visualization to the new structure

This also adds minimal tests for the view
Migrates 'Token Frequency Histogram' visualization to the new structure

Adding some tests
Organizes apps/core/urls.py
Fixes bug introduced in 2dea4fab370
Fixes links to other formats in templates
Migrates 'Word cloud' visualization to the new structure

Also adding some tests
Migrates 'Statistics' visualization to the new structure
Creates a visualization router view so we only have one url